The technical sector of the invention is the production of tools for gripping parts comprising at least one bore or a cylindrical pin.
One of the principal applications of the invention is the dimensional inspection of parts of small dimensions and of complex shape, such as for example those entering in the manufacture of electric circuit breakers. These parts are in fact delivered in large quantities and in batches: in order to be able to inspect dimensioning, twenty or thirty parts in each batch are sampled and certain dimensional characteristics thereof are checked on a three-dimensional measuring machine.
This checking operation requires that the parts be localized with precision, viz. at less than 0.01 millimeters with respect to a referential system of the machine, but also that they be maintained to ensure reliable acquisition of the points of measurement. It is therefore necessary to ensure simultaneous localization and hold of several parts in order to effect the campaign of measurement without the intervention of the operator. The parts in question present complex shapes but all have a reference bore with a diameter of 2 to 5 millimeters.
Now, the various techniques of localization and hold of the parts have developed only little in industry, whether it be in the applications of machining, assembling or inspection. In parallel to these techniques, the use of the methods of standardized dimensioning has demonstrated the usefulness of simple reference surfaces to define a mechanical part dimensionally. At all stages of development of a product, from the study, then manufacture, inspection and up to assembly, the reference surfaces appear as fundamental in the quality thereof. Up to the present time, various gripping systems make it possible to localize and hold mechanical parts, but when the reference surfaces are of cylindrical type (pin or bore), these devices are complex (gripping pliers for solid pin) and expensive, or unsuitable (smooth guiding pin or one with grooves) when the part must also be held. Moreover, the devices for localization in a bore introduce a functional clearance which is often incompatible with the precision of positioning required.
Various specific equipment has been developed for different types of particular parts to be held and of which certain have formed the subject matter of Patent Applications: mention may be made for example of Patent DE 3938150 on a precise localization device for the machining or grinding of disc-shaped parts comprising one or more radial grooves with or without a central hole; the device comprises fingers for positioning in said grooves.
Patent DD 288785 is also known, which discloses an element for positioning and holding flat parts provided with openings and intended to be machined, said element having a cylindrical base form housed on one side in the opening in the part and on the other side in a hole in the frame and made of elastomer of sufficient rigidity, or Patent EP 0474397 which teaches a device for fixing parts on a base having a protusion, and a hole thanks to a rotational element insertable in this hole and an internal expanding mechanism blocking the device in the hole depending on the desired position of blocking of the part.
Numerous other Patents relating to various systems for centering and positioning parts may be mentioned, but it is noted in all these existing devices that the dimensions, the diversity and complexity of the parts make it necessary to produce special gripping tools generally constituted by pliers, metallic guides, screw-clamping devices, etc., which represents study and production costs incompatible with the basic cost of the part in question, especially when the latter forms part of batches manufactured in mass-production as in the application mentioned hereinabove. Whether it be in complex systems or in simple systems requiring in that case multiple points of abutment and anchoring on the part to be held, it is also ascertained that the functions of localization and of hold are not always performed perfectly and create problems of quality of the inspection.
In the particular domain of inspecting parts of small dimensions on three-dimensional measuring machines, numerous problems have, up to the present time, thus remained without a solution. In fact, the weak points of the existing devices are, generally, as follows:
they offer few possibilities of adaptation and flexibility, making it necessary to multiply the number of tools, viz. at least one per part; PA1 their production costs are fairly high and their implementation time long; PA1 it is very difficult to make all the dimensional inspections of a part on the same tool without changing the position of said part due to the difficulties of access to the point of measurement, especially when several anchoring points are necessary; PA1 certain existing devices do not really respect the geometry of the part especially when the holding device requires considerable clamping efforts; PA1 when a bore is taken as point of reference of the measurement, clearances are introduced, rendering said measurement unreliable due to the tolerances of the parts; PA1 it is difficult and sometimes even impossible to industrialize such devices due to their specialization and complexity. PA1 for the users of three-dimensional measuring machines (called TMM): mono- or multi-spindle device for localization and hold of parts presenting a bore or reference shaft of diameter 2 mm; more than 90% of the mechanical parts enter in this category; PA1 for the users of assembly or manufacturing robots: gripping and positioning device for replacing clamps of robots when the parts to be manipulated present a bore or a shaft (90% of the cases); PA1 for aeronautic assemblings: development of independent expandable spindles for the localization and hold of the sheet metal coatings during the counter-boring operations of the structures. PA1 localization by radial concentric expansion, either outside of the finger of the spindle, or inside by radial concentric narrowing of its bore, making it possible to localize a mechanical part with precision, also ensuring hold thereof; PA1 conservation of the perpendicularity of the axis of the finger of the spindle or of its bore with respect to the reference plane formed by its support, during pressurization; PA1 concentricity of the expandable part of the finger of the spindle or its bore with respect to its initial axis after pressurization; PA1 creation of a couple for holding the parts to ensure a stable positioning thereof during the interventions of manufacture or of inspection by the combination of the coefficient of friction of the surface of the parts with those of the device, and of the contact pressure due to expansion; PA1 compensation of the variations in diameter of the bore or the shaft of the parts within the limit of elastic deformation of the device; PA1 tightness of the expandable walls of the spindle which may be of acetal, and reliability due to the use in the elastic domain of deformation; this spindle may be produced by machining: in this version, the metallic envelope is glued on the acetal; or by moulding, in which case the metallic envelope is introduced in the mould before the moulding operation, which ensures a perfect bond of the two materials; PA1 geometry of the assembly ensured by the metallic envelope ensuring the part interface, which makes it possible to obtain high contact pressures with the part to be localized without local deformation (rigidity of the bond) and which, due to its tubular structure, makes it possible to take up the efforts of torsion induced by the outside forces acting on the part, while allowing a sufficient expansion to take into account the variations in diameter of the parts (tolerance on shafts and/or bores) thanks to the longitudinal slots or grooves or slits.